Heroin (diacetylmorphine / diamorphine) is a powerful Opioid that comes in a variety of different preparations, almost exclusively used, today, for recreational use and withdrawal avoidance. Heroin is often considered one of, if not the most, addictive and dangerous recreational drugs available. This page will not attempt to detail all (or even most) information regarding heroin, as that can easily be found elsewhere. This page will focus on popular misconceptions, harm reduction, scientific literature and stats on use.
Additions needed for this article:
- Statistics on addiction rates.
- Citations for all citations needed marks.
- Statistics on overdose by ingestion method.
- Percentage of users who overdose.
- Places to get clean gear in states that do not sell it or operate needle exchanges.
- Weekend Warrior information and studies.
- Lots more information on addiction, like best practices for coming off and stats.
- Information on supervised injection sites and policy regarding them in the U.S.
- Info on different kinds of disinfectant.
Addiction: Psychological vs. Physical
Physical addiction, defined here as experiencing physical withdrawal symptoms after cessation of use, is what most people think about when discussing heroin addiction and heroin addicts.
Psychological addiction, defined here as a psychological need for heroin, is also something that should be taken into consideration, as it can easily lead to relapse and a need for the drug. Psychological addiction is often underestimated. 
Harm Reduction is the practice of teaching addicts the proper way to use with the intention of avoiding medical costs, the spread of disease, and death among addicts and among the general population. Studies have shown that harm reduction is economically more sustainable.  It is also more humane and reduces risk for everyone, because it reduces the amount of users who contract diseases such as HIV / Aids. 
The Hospital, 911, and Naloxone
If someone you know is experiencing an overdose, the best course of action is to either administer Naloxone, call 911, or quickly get said person to the hospital. You can get in much more legal trouble for not getting help than you can for saving someones life. Besides, it's obviously the right thing to do. If you are using heroin, you should be mentally prepared to face the consequences of said use, and prepared to risk a few nights in jail to save someones life.
You can always claim you didn't know the person was using and you were not in the room when it occurred. If you absolutely refuse to take this "risk", the least you can do is transport the overdose victim to a sidewalk, and call 911 to report the overdose from there. This wastes precious time, however, and again, the best thing to do is contact 911 right away. You may also wish to begin CPR, if you know how to do it correctly. Consider learning CPR or finding a source for Naloxone if you are going to continue to use heroin. Avoid attempting to inflict pain to bring the person back to consciousness. Place your knuckles on their sternum, and rub them around with some pressure. If this doesn't bring them around, chances are they won't be waking up.
(Note, This part really needs a lot of research to back it up, I just went with what I remember at the moment, but I'll have to find the sources for all of this before I'm confident in keeping this here unchanged.)
Using heroin in the most sanitary way possible can be the difference between collapsed veins, contracting STI's / STD's, contracting AIDS, developing abscesses, possible respiratory diseases, damaged nasal tissue, developing track-marks, possible overdose and the minimization of danger and relatively safe usage. While using heroin will always retain risk, if one decides to use it, then the smart user is the one who does the research and uses as safely as possible.
Intravenous Injection can be extremely risky. It introduces a usually unverified substance with adulterants used to "cut" the dope so the dealers can sell more, and reduce the purity. Granted, if they didn't, users would be OD'ing left and right, but depending on what they use to "cut" their dope with, it could introduce unwanted substances into your bloodstream. The best way to combat this, and other biological and viral infections, is to make sure you use completely new syringes, use a micron filter if possible to filter out as much as possible, swab down the injection site with alcohol prior to injection, and swab down the spoon with alcohol as well. The idea here is to kill off as many bacteria and viruses as possible. Never share a needle, and try to avoid reusing needles. Needle reuse, even if it's you who you're using it after, can be very dangerous. Microscopic bits of blood can develop dangerous bacteria over time, which can lead to infection. If you absolutely must reuse a needle, at the very least clean it out with alcohol. It's better to use bleach, however, and even then some viruses may not be totally killed off. 
(Note: Avoid using these tips until proper citations have been provided.)
Just like Intravenous, you want to avoid sharing any of your works. Do not share straws, and avoid using money to snort. Get a pack of clean straws, and cut them up. Discard them after you are done. Avoid reusing the same straw. You should also avoid snorting off of a dirty surface. Swab down a mirror with alcohol to help kill of bacteria and other infectious microbes. Clean anything you are using to snort with thoroughly before and after each session. It should also be noted that you should make sure your heroin is as thin as possible while snorting, this is easier on the nose in the long run. 
Smoking is another possible method. Users will generally put their tar or powder on foil, then light underneath of this and try to suck up the smoke. This is known as "chasing the dragon". Again, avoid sharing utensils. Some users suggest "sanitizing" the foil, because foil can be extremely toxic when burned. 
Obtaining the Works.
Depending on where you live, obtaining clean gear can either be as easy as walking into wall-mart and asking for a pack for $1.25, to being forced to forge a script, find some online, or travel. As discussed above, you should always buy fresh needles. There is no safe substitute. Generally, if you find a place to buy fresh needles, it's best to buy in bulk rather than buy in individual packs so as to avoid repeated trips. Those buying in packs of ten generally seem more like users just trying to get their fix than those buying in bulk for insulin shots.
If you can find a pharmacy that will sell to you, the best way to go is to go prepared. Know what gauge and length you want. Generally, you'll want a guage anywhere from 31 to 28. The larger the gauge number, the smaller the actual needle is. You'll have to experiment and decide for yourself which size you prefer. Larger syringes generally stay in place more easily, but leave a bigger mark, smaller are harder to keep steady, but leave less of a mark. As far as the length goes, they usually come in 1/4th inch, 1/2", and 1". 1/2" is generally suitable for intravenous injections.
If they ask why, either say you are diabetic or buying them for someone close to you who is diabetic. Sometimes it's better to bring a note with what you want on it, as if you're coming in to pick it up for someone else. You can generally find information on weather your area has a pharmacy that will sell you syringes by searching online to check the local and state or regional laws in your area. If you can find a needle exchange in your area, opt for that. They will usually give you free syringes, along with a variety of other bits and pieces that are far better suited for the job than the standard setup.
Basically, if you can't get access to micron filters, you use cotton. Micron filters are very much preferable, however, as they filter out bacteria and other junk that may make it's way through cotton, and directly into your bloodstream.  If you can't get micron-filters, however, cotton will have to suffice. Most users make use of cotton. Make sure you are getting fresh cotton balls, and avoid using anything like a cigarette filter or older cotton that's been sitting out for a while. Cigarette filters have tiny shards of glass and can contain other toxins which you do not want to be injecting.  Cotton is not ideal, but it's better than nothing, and will filter out quite a bit of extra grime that you do not want in your system. One downside cotton has is the possibility of cotton fever, which is a mostly benign syndrome caused by cotton from cotton plants heavily colonized with Enterobacter agglomerans. Symptoms usually include a fever, chills, a headache, among others. Keep in mind, however, that if cotton fever does not subside within a day, it may be another serious infection and you should contact a medical professional right away.
You should have either alcohol wipes, or isopropyl alcohol available to sanitize an injection site before you consider injecting. This can help reduce bacteria and viruses which can get into your bloodstream from your skin. It can also help reduce the chance of developing an abscess in the event that you miss your shot.